The German justice minister has told a German newspaper that he assumes the crimes that took place in Cologne during New Year celebrations were "coordinated or prepared", as police say the number of sexual assaults and robberies reported has risen significantly.

In a statement released on Sunday, Cologne police said that they were investigating 516 criminal complaints and that about 40 percent of those allegations were of sexual offenses.

"If such a horde gathers in order to commit crimes, that appears in some form to be planned," Justice Minister Heiko Maas told the Bild newspaper. "Nobody can tell me that this was not coordinated or prepared.

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"All connections must be carefully checked," Maas said. "There is a suspicion that a particular date was chosen with expected crowds. That would then be a new dimension."

Maas said he wanted to quickly determine whether the New Year's assaults in Cologne could be linked to similar offenses in other cities such as Frankfurt, Stuttgart and Hamburg, where women also claimed to have been harassed.

Dozens of women were assaulted or robbed during the New Year's night in Cologne, the police announced last Monday. In the days that followed, many more cases were reported.

Police confirmed on Sunday that the number of reported assaults in Cologne are still rising.

"The rising numbers also have a geographic reason: Some women were in Cologne for New Year's Eve but don't actually live here," Dirk Weber, a Cologne police officer, told Al Jazeera.

Further investigations

Authorities and witnesses said the attackers were among about 1,000 men who gathered at Cologne's central train station, some of whom broke off into small groups that groped and robbed women.

The attacks are still being investigated, but police have said that their focus was on suspects of primarily North African origin, which has put pressure on Chancellor Angela Merkel's government and its open-door policy to asylum seekers.